Integrated circuit chips are well known in the art. The microprocessor chip is, perhaps, the most familiar of these chips and the invention, accordingly, will be described in terms of such a chip.
A microprocessor is characterized by a datapath in which various mathematical operations are executed. The operations typically are carried out by the manipulation of data stored in memory frequently external to the chip. The data in memory is identified and moved to on-chip storage areas in accordance with instructions also stored in external memory. The instructions are applied to an instruction register often for inputting, for example, to a programmable logic array (PLA). The PLA, in turn, outputs sequences of microcode (machine instructions) for manipulating the elements of the datapath.
Transmission of data between various elements of the microprocessor is by multiple conductors arranged in bus structures as is well understood. The bus over which data moves in the microprocessor is called the databus. The databus outputs signals via an input/output (I/O) interface which frequently takes the form of a frame about the edge of the chip.
The I/O frame includes a number of (high capacitance) circuits operative to shape outputs from the databus so that the signals are useful for driving external circuitry. Such a circuit may include, for example, a plurality of inverters connected, electrically in series, between the databus output and a pad to which external connection may be made. The circuits in the I/O frame are clocked by pulses on clock distribution lines as are the control, logic and datapath elements in the microprocessor. It has been found to be difficult to avoid clock skew in the circuits of the I/O frame. Consequently, the throughput of microprocessors is relatively limited to the extent that skew is not reduced.